1. Field of the Invention
The present invention is directed to a gradient coil arrangement for a magnetic resonance tomography apparatus, for generating transverse or longitudinal gradient fields.
2. Description of the Prior Art
The required performance capability of a gradient coil is essentially dependent on the type of MR imaging. Conventional MR imaging usually requires a good linearity volume (-5% in the linearity volume of 40-50 cm) with moderate gradient strength (10-20 mT/m) and switching times (-1 ms). For fast MR imaging, high amplitude gradients (20-40 mT/m) are switched very fast (100-500 .mu.s). As a result, side affects in the form of peripheral muscle stimulations can occur. In order to avoid these effects, the linearity volume of the gradient coils is generally reduced, which leads to a reduction of the maximum field boosts, and thus also leads to a reduction of the stimulation risk (the maximum field boost, in addition to other aspects, determines the stimulation risk). Given rapidly switched gradient coils, the linearity volume can thus diminish very quickly from, typically, 40-50 cm to 20 cm DSV. A coil having such properties is usually not suited for conventional whole-body applications, but is suitable for fast MR imaging techniques such as EP, RARE, HASTE, GRASE, etc. The speed is the important advantage.
Another reason for different field qualities is that the linearity generally decreases with the distance from the center when a gradient coil is designed for a specific volume. The human body, however, does not necessarily follow this rule. For example, the shoulders are located in this region. Given exposures of the spinal column, it is often meaningful to image the entire spinal column without repositioning. Dependent on the positioning of the center, the cervical and/or lumbar vertebra lie in the region of the greatest non-linearities. Image distortions are therefore unavoidable. Due to the smaller diameter of the coil, there is a smaller homogeneity volume for head gradient coils. This only allows the imaging of parts of the brain but not the imaging of the cervical spinal column. Therefore it can be desirable for the radiologist to switch from a central FOV to a displaced FOV. This, however, has not been hitherto possible. Only embodiments of the one or other type exist.
In order to avoid defining the field quality that the gradient coil arrangement should have at the time of manufacture which would result in an inflexible system unable to meet the differing needs of various customers, a magnetic resonance imaging system is disclosed in German OS 195 40 746 wherein a modular gradient system is employed, which combines a conventional and a fast gradient coil system in one coil body. The conventional gradient system has a large linearity volume that, however, can only be slowly switched and, moreover, only allows medium gradient amplitudes. The fast gradient system, by contrast, exhibits a smaller linearity volume but instead allows faster switching of very high gradient amplitudes. Fundamentally, however, this is nothing more than the combination of two completely separate gradient coil systems that are merely wound on the one and the same tubular carrier, with a series connection or parallel connections also being possible in addition to the separate drive of these gradient coil systems.
U.S. Pat. No. 5,349,318 discloses a gradient coil arrangement wherein conductors of the gradient coil are arranged essentially in a primary plane, which is an inner cylindrical envelope, and in a secondary plane, which is an outer cylindrical envelope which concentrically surrounds the inner cylindrical envelope. Each conductor arrangement of the respective two cylindrical envelopes contains a sub-coil having a helical conductor arrangement as well as a number of sub-coils having a horseshoe-shaped conductor arrangement. The open conductor ends of the sub-coils are firmly connected to one another, via conductive connecting wires, at an end side between the two cylindrical envelopes, the connecting wires being, for example, soldered to the coil conductor ends. The field quality of the gradient coil arrangement is thus also defined and is invariable.